Glycogen synthase kinase-3 (GSK-3) is a serine/threonine protein kinase comprised of α and β isoforms that are each encoded by distinct genes [Coghlan et al., Chemistry & Biology 2000, 7, 793-803; and Kim and Kimmel, Curr. Opinion Genetics Dev., 2000 10, 508-514]. GSK-3 has been implicated in various diseases, disorders, and conditions including diabetes, Alzheimer's disease, CNS diseases such as bipolar disorder, schizophrenia, cerebral stroke, Huntington's and other neurodegenerative diseases, leukocytopenia and cardiomyocyte hypertrophy [PCT Application Nos.: WO 99/65897 and WO 00/38675; and Haq et al., J. Cell Biol. 2000, 151, 117-130]. These diseases, disorders, and conditions are associated with the abnormal operation of certain cell signaling pathways in which GSK-3 plays a role. GSK-3 has been found to phosphorylate and modulate the activity of a number of regulatory proteins. These proteins include glycogen synthase, which is the rate limiting enzyme necessary for glycogen synthesis, the microtubule associated protein Tau, the gene transcription factor β-catenin, the translation initiation factor e1F2B, as well as ATP citrate lyase, axin, heat shock factor-1, c-Jun, c-myc, c-myb, CREB, and CEPBα. These diverse protein targets implicate GSK-3 in many aspects of cellular metabolism, proliferation, differentiation, and development.
In a GSK-3 mediated pathway that is relevant for the treatment of type II diabetes, insulin-induced signaling leads to cellular glucose uptake and glycogen synthesis. Along this pathway, GSK-3 is a negative regulator of the insulin-induced signal. Normally, the presence of insulin causes inhibition of GSK-3 mediated phosphorylation and deactivation of glycogen synthase. The inhibition of GSK-3 leads to increased glycogen synthesis and glucose uptake [Klein et al., PNAS 1996, 93, 8455-8459; Cross et al., Biochem. J. 1994, 303, 21-26); Cohen, Biochem. Soc. Trans. 1993, 21, 555-567; and Massillon et al., Biochem J. 1994, 299, 123-128]. However, in a diabetic patient, where the insulin response is impaired, glycogen synthesis and glucose uptake fail to increase despite the presence of relatively high blood levels of insulin. This leads to abnormally high blood levels of glucose with acute and long-term effects that may ultimately result in cardiovascular disease, renal failure and blindness. In such patients, the normal insulin-induced inhibition of GSK-3 fails to occur. It has also been reported that in patients with type II diabetes, GSK-3 is overexpressed [see, PCT Application: WO 00/38675]. Therapeutic inhibitors of GSK-3 are therefore potentially useful for treating diabetic patients suffering from an impaired response to insulin.
GSK-3 activity is associated with Alzheimer's disease. The hallmarks of this disease are the extracellular plaques formed by aggregated β-amyloid peptides and the formation of intracellular neurofibrillary tangles via the tau protein.
It has been shown that GSK-3 inhibition reduces amyloid-β peptides in an animal model of Alzheimer's disease. See pages 435, 438. Phiel et. al., Nature 423, 435-439 (2003). Mice over-expressing amyloid precursor protein (APP) treated with lithium (a GSK-3α inhibitor) over a three-week period showed over a 50% decrease in amyloid-β peptide tissue levels.
The neurofibrillary tangles contain hyperphosphorylated Tau protein, in which Tau is phosphorylated on abnormal sites. GSK-3 is known to phosphorylate these abnormal sites in cell and animal models. Conditional transgenic mice that over-express GSK-3 develop aspects of AD including tau hyperphosphorylation, neuronal apoptosis and spatial learning deficit. Turning off GSK-3 in these mice restores normal behavior, reduces Tau hyperphosphorylation and neuronal apoptosis. (Engel T et al., J Neuro Sci, 2006, 26, 5083-5090 and Lucas et al, EMBO J, 2001, 20, 27-39) Inhibitors of GSK-3 have also been shown to prevent hyperphosphorylation of Tau in cells [Lovestone et al., Current Biology 1994, 4, 1077-86; and Brownlees et al., Neuroreport 1997, 8, 3251-55].
GSK-3 as a target for psychosis and mood disorders, such as schizophrenia and bipolar disease, respectively, have been reported in the literature. AKT haplotype deficiency was identified in a subset of schizophrenic patients which correlated with increased GSK-3 activity. A single allele knockout of GSK-3β resulted in attenuated hyperactivity in response to amphetamine in a behavior model of mania.
Several antipsychotic drugs and mood stabilizers used to treat both schizophrenic and bipolar patients have been shown to inhibit GSK-3 (Emamian et al, Nat Genet, 2004, 36, 131-137; Obrien et al, J Neurosci, 2004, 24, 6791-6798; Beaulieu et al, PNAS, 2004, 101, 5099-5104; Li et al Int J Neuropsychopharmacol, 2006, pp 1-13; Gould T D, Expert Opin Ther Targets, 2006, 10, 377-392). Furthermore, a recent patent, US 2004/0039007 describes GSK-3 inhibitors that show anti-schizophrenic and anxiolytic effects in relevant mouse behavior models.
GSK-3 activity is associated with stroke. Wang et al. showed that IGF-1 (insulin growth factor-1), a known GSK-3 inhibitor, reduced infarct size in rat brains after transient middle cerebral artery occlusion (MCAO), a model for stroke in rats. [Wang et al., Brain Res 2000, 859, 381-5; Sasaki et al., Neurol Res 2001, 23, 588-92; Hashimoto et al., J. Biol. Chem 2002, 277, 32985-32991]. US 2004/0039007 describes the effect of GSK-3 inhibitors in MCAO, a stroke model in rats. These GSK-3 inhibitors significantly reduced striatal ischemic damage and reduce edema formation in rats. Additionally, the rats “demonstrated marked improvement in neurological function over the time course of the experiment.”
Inhibition of GSK-3 activity has been linked to stem cell proliferation, differentiation and neuronal plasticity Inhibitors of GSK-3 have been shown to sustain self-renewal of embryonic stem cells, promote neuron, beta-cell, myeloid and osteoblast differentiation. (Sato et al, Nature Medicine 10, 55-63, 2004; Ding et al PNAS 100, 7632-37, 2003; Branco et al J Cell Science 117, 5731-37, 2004; Trowbridge et al, Nature Medicine 12, 89-98, 2006; Mussmann et al, JBC (Epub ahead of print) 2007; Kulkarni et al Journal of Bone and Mineral Res. 21, 910-920, 2006) With respect to neuronal plasticity, inhibition of GSK-3 has been shown to be important for regulating polarity, long-term potentiation (LTP) and neurite/axon growth (Hooper et al European J of Neuroscience 25, 81-86, 2007; Kim et al, Neuron 52, 981-996, 2006; Jiang et al Cell 120, 123-135, 2005). Taken all together, GSK-3 small-molecule inhibitors have the potential to act as chemomodulators of cell differentiation and plasticity which has implications for many types of degenerative conditions such as Neurodegenerative diseases (Stroke, Alzheimer, Parkinson, Huntington, ALS and Multiple Sclerosis), Leukocytopenia, Diabetes and Osteoporosis.
Accordingly, there is a great need to develop compounds useful as inhibitors of protein kinases. In particular, it would be desirable to develop compounds that are useful as inhibitors of GSK-3, particularly given the inadequate treatments currently available for the majority of the disorders implicated in their activation.